Conventionally, in a mobile communication system, a state of a propagation channel (path) between a transmitter and a receiver is accurately estimated, and a received signal of the receiver is equalized by using a propagation channel estimation value as a result of estimation, so that the signal is compensated for the distortion occurred in the propagation channel. In addition, in the case where a change in the propagation channel is slow at the time of estimating the propagation channel estimation value (for example, in the case where the communication method of the mobile communication system is an orthogonal frequency division multiplexing (OFDM) method, the receiver performs calculation under the assumption that the propagation channel estimation value of each of the sample signals in one symbol is constant.
A conventional propagation channel estimation method will be described. A transmitted signal of the time t is denoted by x(t); a noise is denoted by n(t); and the propagation channel estimation value of a delay time τ of the time t is denoted by h(τ, t). In this case, the received signal y(t) of the time t can be expressed by the following equation.y(t)=h(τ, t)x(t)+n(t)  (1)where  denotes convolution operation. Therefore, in a communication using a spectrum spread method, the propagation channel estimation value is estimated by the following equation.
                                          h            ^                    ⁡                      (                          τ              ,                              t                +                                  T                  /                  2                                                      )                          =                              ∑                          t              =              0                                      T              -              1                                ⁢                                    y              ⁡                              (                t                )                                      ⁢                                                            x                  ^                                *                            ⁡                              (                                  t                  -                  τ                                )                                                                        (        2        )            where {circumflex over (x)}*(t) denotes a complex conjugate of a known transmitted signal replica x(t), and T denotes the number of samples included in one symbol. In addition, in a communication using the OFDM method, the propagation channel estimation value is estimated by the following equation.ĥ(c, t+T/2)=FFT[y]/FFT[{circumflex over (x)}]  (3)where c denotes a subcarrier, and y denotes a sequence from a time sequence y(t) corresponding to one symbol to a time sequence y(t+T−1). As expressed in the above Equations (2) and (3), in a conventional propagation channel estimation method, in the case of the signals corresponding to the T sample signals from y(t) to y(t+T−1), the propagation channel estimation value of the time t+T/2 is estimated, and the propagation channel estimation value is used as the constant value of each of the samples in one symbol.
However, recently, since the frequency band of the mobile communication system is heightened, influence of a frequency error between the transmitter and the receiver and Doppler shift associated with movement of the transmitter and the receiver becomes notable, so that a change in the propagation channel is fast.
However, in the conventional propagation channel estimation method, since the propagation channel estimation value at the time t+T/2 of the center of one symbol, that is, T sample signals are used as the constant value in one symbol, in the case where the propagation channel is changed within one symbol, that is, within the T sample signals, due to the fast change in the propagation channel, an accurate propagation channel estimation value cannot be obtained. Therefore, the received signal cannot be accurately equalized.
Under such circumstances, various technologies using an adaptive algorithm capable of adaptively updating the propagation channel estimation value even in an environment of the propagation channel of which a change is fast have been considered.
As such types of the technologies, there are technologies of performing threshold value determination on tap coefficients (corresponding to a propagation channel estimation value) after correction (updating) according to an adaptive algorithm to remove tap coefficients corresponding to noise and performing equalization of a received signal by using only the remaining tap coefficients (for example, refer to Japanese Laid-open Patent Publication Nos. 11-313013, 2005-51404, and 2005-51404) and technologies of performing threshold value determination on an equalized signal by using tap coefficients after updating according to an adaptive algorithm (for example, refer to Japanese Laid-open Patent Publication No. 05-308252)
However, in all the conventional technologies disclosed in Japanese Laid-open Patent Publication Nos. 11-313013, 2005-51404, 2005-51404, and 05-308252, all the signals propagating through a plurality of propagation channels between the transmitter and the receiver are input to the adaptive algorithm, and the propagation channel estimation values for all the propagation channels are calculated through calculation of the adaptive algorithm. Therefore, the calculation amount of the adaptive algorithm is very large, so that a speed of calculation of the propagation channel estimation values is delayed.
In addition, after the threshold value determination is performed on the tap coefficients after the updating according to the adaptive algorithm, the propagation channel estimation value corresponding to the tap coefficients each of which is equal to or smaller than the threshold value, that is, the noise component is forcibly set to zero, so that the accuracy of calculation of the propagation channel estimation value is decreased.